This invention relates generally to a substrate for dissipating heat from an integrated circuit device, and more particularly, to a method for mounting such a substrate to a PC board.
As integrated circuits ("ICs"), and in particular, microprocessors are designed to run at increasing speeds, such circuits consume more power and generate more heat. Such heat needs to be dissipated to maintain the integrated circuit and nearby circuits within rated operating temperature ranges. Conventionally, this is done by coupling the IC's to heat sinks and blowing air using cooling fans. In addition, some faster microprocessor chips, such as some Pentium.TM. microprocessor models, have increased maximum temperature tolerances to 95.degree. C. Although such chips can withstand increased and relatively high operating temperatures, neighboring IC's can not The neighboring chips are often limited to conventional maximum operating temperatures of approximately 70.degree. C. With the microprocessor in close proximity to these less tolerant components, some of the microprocessor's heat tends to flow into the printed circuit ("PC") board and to such components. Of concern is the overheating of the surrounding components. To avoid damage to the less tolerant components in the vicinity of the heat generating microprocessor, it is known to throttle back the microprocessor's speed when peak operating temperatures are sensed. Reducing the speed causes the microprocessor to generate less heat. It also means, however, that the customer's product is not operating as fast as expected. Accordingly, there is an ongoing need for transferring heat away from the microprocessor without unduly heating neighboring components and without compromising processor speed.
A conventional heat transfer method uses copper-lined vias in a PC board to transfer heat from the microprocessor to a heat sink. Copper is a good conductor of heat. The microprocessor is mounted to the PC board providing a thermal contact between the microprocessor and the copper walls of the vias. A heat sink typically is mounted to the underside of the PC board. Heat from the microprocessor then is conducted along the copper walls away from the microprocessor to the heat sink. A shortcoming of such approach is that heat conducted along the copper walls of the vias travels, not only to the heat sink, but also into the several copper layers of the PC board. The PC board typically includes a plurality of copper layers for electrically interconnecting the multiple IC chips on the PC board. Such copper layers border the via at the copper lining. Thus, undesirably effective heat transfer paths also occur between the microprocessor and nearby devices.
Another shortcoming of conventional heat transfer solutions relates to the microprocessor surface mounting process. To enhance thermal conductivity between a microprocessor and the PC board vias, the region where the microprocessor is to be placed typically is covered with solder. During the surface mounting process, the solder becomes liquid. Then, when resolidifying, the solder defines a heat conductive bridge from microprocessor to PC board. Unfortunately the solder mounds up in areas resulting in a very uneven surface. The non-planar solder interface between microprocessor and PC board reduces the thermal transfer effectiveness of the bridge. Accordingly, there is need for a more effective chip mounting technique, and a more effective thermal contact.